The invention relates to digital data transmission techniques and its application in digital mobile phone systems.
Bit errors occur in the speech blocks of digital data transmission and it is not possible to correctly decode all the blocks at the receiving end. Therefore, some method must be used to remove faulty speech frames in order to avoid the disturbing sounds in earphones or speakers that are caused by the faulty frames.
GSM speech channels (GSM, Group Special Mobile) use three parity bits in each speech frame for detecting errors in data transmission. However, this is not sufficient. In addition to the parity check another method for estimating the quality of the received signal should be used. If the estimated quality of the signal is poor, the speech frame is removed even though the parity was correct. It should be remembered that the parity can be correct even though the only air interface signal received is noise.
There are two quality measures that are used in respective methods for estimating the quality of the received signal ie the estimation of the signal-to-noise ratio, and the estimation of the bit error rate of a decoded channel. The respective quality measures for these methods are the Estimated Signal-to-Noise Ratio (ESNR) and the Pseudo Bit Error Rate (PBER).
When using the method using the signal-to-noise ratio, a poor-quality speech frame is detected by giving the ESNR a certain threshold value. In this case all frames with an ESNR lower than the threshold value are removed.
When using the bit error rate method, the decoded speech frame is recorded and the coded bits are compared with the received bits. The PBER which is an estimate of the real bit error rate is obtained in this way. A certain threshold value can also be placed on the number of bit errors, whereby all the frames with a higher number of bit errors are removed.
Neither of the above-mentioned known methods is completely reliable. When using threshold values, some good frames are removed in both methods and some poor frames remain untouched.